Nancy Reagan welcomed 450 wives of members of the American Association of Neurological Surgeons (AANS) to the White House yesterday and, for her hospitality, received a $5,000 check for the Loyal and Edith Davis Professorship of Surgery at Northwestern University Medical School. The gift will go toward the research chair, which was established last May with a projected endowment goal of $1 million.

"You can't possibly know how much this means to me," the first lady said, her voice breaking with emotion as she accepted the check from Betty Wrenn, wife of Dr. Fred Wrenn, AANS president. An estimated 2,700 neurosurgeons are meeting in Washington this week.

Before his retirement, Loyal Davis headed Northwestern's department of surgery from 1932 to 1963. He died last year at the age of 86. Edith Davis, 86, who is in failing health, lives in Arizona.

"It was so typical of him that he did not want this chair just named after himself," Mrs. Reagan said. "He wanted to acknowledge my mother's contribution to him, his work, his life and everything she did for him. She was very much a part of his life."

The first lady said that as a young girl watching her father perform surgery at Passavant Hospital in Chicago, "I can remember . . . feeling so proud every time he came into the operating room and watching his hands, his fingers tying off these tiny little things I couldn't even see."

She said that when he permitted her to watch him, she felt she had passed "some kind of test--he knew I wasn't going to faint and disgrace him. One time he let me stand in the operating room, and I really felt I'd passed the test."

Arrangements chairman for the White House tea was Cindy Kobrine, whose husband, Arthur, was the neurosurgeon for White House press secretary James Brady, wounded in the assassination attempt on President Reagan in 1981.

Kobrine said auxiliary members had "preregistered" several months ago for the tea and that all who attended also had sent contributions to the Loyal and Edith Davis chair.

"We only asked for $10, but many sent a lot more. We just said, 'We're offering this tea for free, but what we'd like to do is give Mrs. Reagan a donation to her father's chair,' " Kobrine said.

She said that since the auxiliary is not a service organization, members have a difficult time finding worthwhile things to do every year.

"What more special thing could you do," she asked, "than something for Nancy Reagan's father?"